The Swiss lever escapement is a very widely used device which forms part of the regulating member of mechanical watches. This mechanism makes it possible to simultaneously maintain the motion of a sprung balance resonator and to synchronise the rotation of the drive train with the resonator.
In order to fulfil these functions, the escape wheel interacts with the pallet fork by means of mechanical contact forces, and the Swiss lever escapement uses this mechanical contact between the escape wheel and the Swiss lever to fulfil a first function of transmitting energy from the escape wheel to the sprung balance on the one hand, and to fulfil on the other hand a second function which consists of releasing and locking the escape wheel in jerks so that it advances by one step at every vibration of the balance.
The mechanical contacts required to accomplish these first and second functions impair the efficiency, the isochronism, the power reserve and the working life of the watch.
Different studies have proposed synchronising the rotation of the drive wheel with a mechanical resonator by using a contactless force, such as “Clifford” type escapements. All of these systems use an interaction force of magnetic origin that allows for the transfer of energy from the drive wheel to the resonator at the rate imposed by the natural frequency of the resonator. However, they all suffer from the same drawback of failing to fulfil the second function of releasing and locking the escape wheel in jerks in a reliable manner. More specifically, following a shock, the wheel may be desynchronized from the mechanical resonator, and as a result the regulating functions are no longer ensured.
U.S. Pat. No. 3,518,464 in the name of KAWAKAMI TSUNETA describes an electromagnetic mechanism for driving a wheel by a resonator. This Patent mentions that the use of a magnetic drive mechanism as an escapement has an unfavourable effect on frequency. This mechanism includes a vibrating strip, but no stop member, and certainly no multi-stable stop member. During rotation of the wheel and in a fixed position of the resonator, the force between the wheel and the resonator varies progressively between a minimum (negative) and a maximum (positive) value over an angular period.
DE Utility Model No. 1935486U in the name of JUNGHANS describes a drive mechanism with magnetic detents. This mechanism also includes a vibrating strip, but no stop member, and certainly no multi-stable stop member. This mechanism includes ramps and barriers which make use of combined and simultaneous movements of the wheel and the resonator.
U.S. Pat. No. 3,183,426A in the name of HAYDON ARTHUR describes an entirely magnetic escapement including a magnetic escape wheel, in which the energy varies continuously and progressively between minimum and maximum when the wheel turns through one half-period and then the energy returns to a minimum value over the following half-period. In other words, the magnetic force on the wheel varies progressively between a minimum (negative) and maximum (positive) value over an angular period.